1. Field of the Invention
The present invention relates to a method for performing joint time synchronization and frequency offset estimation in a high-speed wireless communication system and an apparatus, in particularly, to a method and an apparatus for performing joint time synchronization and frequency offset estimation in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system. The method and apparatus can perform carrier frequency offset acquisition while performing accurate time synchronization, and have an effect of a large carrier frequency offset acquisition range and a high estimation accuracy.
2. Description of the Prior Art
In a high-speed wireless communication system, especially an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system, accurate and reliable time synchronization is essential so that a receiver can properly demodulate data transmitted by a transmitter. Meanwhile, since the OFDM system is very sensitive to carrier frequency offset, it is necessary for the receiver to provide an accurate and efficient algorithm to properly estimate and correct the carrier frequency offset. The receiver has to accurately locate the received data frames, and determine a beginning point and an ending point of each data frame.
However, due to various causes such as miss matching between the frequencies of the oscillators of the transmitter and receiver, Doppler frequency shift etc, frequency offset may occur between the transmitter and the receiver. Further, the frequency offset will also impact on the demodulation performance of the receiver. A conventional method in the existing synchronization system is to achieve time synchronization and frequency synchronization with the aid of a training sequence.
In the existing synchronization algorithms, some ones use a null symbol to perform synchronization location. That is, when the receiver detects a low voltage symbol, it assumes that a new symbol is coming. But this algorithm can only be used in a continuous transmission mode of broadcasting, and will fail in a burst transmission mode like in a wireless local area network. Since the burst transmission system may be in an idle state for a long time period, it cannot be distinguished whether the system is in an idle state or receives a null symbol. Some algorithms use a cyclic prefix in OFDM symbols to perform synchronization. However, this kind of algorithms can only perform symbol synchronization other than frame synchronization. In addition, this kind of algorithm has a lower accuracy due to the short length of common cyclic prefixes. Another kind of algorithm uses the training sequence to perform time synchronization and carrier frequency offset estimation, but the average power of the training sequence used by these conventional algorithms is the same as that of a general data symbol, and therefore it may result in a higher packet loss rate, especially in a condition of low signal-to-nose ratio. In general, also, many existing algorithms perform the time synchronization first and then perform the carrier frequency offset estimation. The disadvantages of this synchronization scheme are in that: since the time synchronization offset and the carrier frequency offset are not independent from each other but influenced, the condition of the carrier frequency offset estimation is that an accurate time synchronization has to be achieved by the receiver. However, the performance of the time synchronization is influenced by the carrier frequency offset. Since they are dependent on each other, individually processing the time synchronization and the carrier frequency offset will cause a large error, and even probably result in the failure of the algorithms.